Traditionally, a four-wheel drive vehicle has been in practical use based on a FF vehicle (a front engine/front drive vehicle) or a FR vehicle (a front engine/rear drive vehicle). For example, a four-wheel drive vehicle based on the FF vehicle is controlled to distribute most of the driving force to front wheels under normal traveling conditions and increase the distribution of the driving force to the rear wheels at the traveling time on a low μ road (a low friction test road). Therefore, four-wheel drive vehicles based on FF or FR vehicles are normally driven under close to two-wheel drive conditions, and are driven by setting the distribution of the driving force between the front wheels and the rear wheels based on road surface conditions or vehicle attitudes, and also input information from a driver. Such a four-wheel drive vehicle can minimize energy loss because it travels by two-wheel drive normally, and also can control wheel skid caused by traveling on such low μ roads using four-wheel drive.
In Patent Document 1, a four-wheel drive vehicle has a longitudinal G (acceleration) sensor and calculates a road surface gradient value based on signals from the longitudinal G sensor. The four-wheel drive vehicle disclosed in Patent Document 1 uses the road surface gradient value that is calculated based on signals from the longitudinal G sensor, and sets a distribution of the driving force between the front wheels and the rear wheels. Accordingly, the four-wheel drive vehicle in Patent Document 1, for example, can control the occurrence of skids when starting from an uphill stance on a low μ road.